1. Field of the Invention
The present invention relates generally to a device and method for adjusting the exposure of an image sensor and, more particularly, to a device and method for adjusting the exposure of an image sensor capable of performing fast exposure correction for various light sources.
2. Description of the Background Art
An image sensor is a device for photographing images using the light-sensitive characteristic of a semiconductor to detect the varying brightness and wavelengths of light reflected from subjects and to convert the detected brightness and wavelength into electrical values for each pixel. Conversion of the electrical values into levels at which signal processing can be performed is a function of the image sensor. In other words, the image sensor is a semiconductor device for converting an optical image into electrical signals.
A Charge Coupled Device (CCD) or CMOS (Complementary Metal Oxide Semiconductor) image sensor is considered a representative example of an image sensor. A CCD image sensor is a device in which individual MOS capacitors are located closely adjacent to each other. The CCD image sensors transfers an optical image into electrical signals by storing and transmitting charges.
In contrast, a CMOS image sensor is a device that uses the CMOS technique of using a control circuit and a signal processing circuit as peripheral circuits and adopts the switching method of sequentially detecting output by forming and using MOS transistors in proportion to the number of pixels. Such a CMOS image sensor has the advantage of low power consumption, so that it is useful for a personal portable device, such as a mobile phone. Accordingly, the CMOS image sensor can be applied to various applications, such as a Personal Computer (PC) camera, a medical camera, or a toy camera.
In addition, since the characteristics of light sources of an image sensor, such as luminance and color temperature, can vary according to the type of light sources, it is necessary to adjust exposure for the light sources in order to maintain proper luminance for the various light sources having different characteristics. Conventionally, the following two methods have been mainly used to adjust the exposure of an image sensor. A first method is a method of determining an appropriate exposure value for a plurality of frames while increasing or decreasing a currently set exposure value for an image sensor at a low rate. This method facilitates the hardware implementation of an exposure adjustment device for an image sensor, but is problematic in that fast correction is difficult because the exposure value must be corrected for a plurality of frames. In particular, a white screen sustains because a plurality of frames is used at a maximal exposure value (generally, ⅕ sec) when an image sensor moves from a dark place to a bright place, so that fast screen switching is difficult.
A second method is a method of measuring current ambient luminances using an auxiliary device, such as a photodiode (for example, a Light Emitting Diode (LED)), storing an appropriate exposure value list based on the measurement in non-volatile memory (for example, Read-Only Memory (ROM), flash memory, etc.), and then referring to the stored list. This method makes fast correction possible, but has problems in that it requires a separate storage device to store the exposure value list and must acquire the experimental exposure values appropriate for ambient luminance. Furthermore, in the case in which a light source having power frequency characteristics similar to those of a fluorescent lamp is used as the light source of an image sensor, attenuated emission occurs due to an intrinsic afterglow characteristic. As a result, so-called flicker results from variation in color as well as brightness.